In a charged particle beam device typified by a scanning electron microscope, a sample is scanned with a finely focused charged particle beam, whereby desired information (for example, a sample image) is obtained from the sample.
In such a charged particle beam device, up to now, observation utilizing reflection electrons with relatively high energy has been the mainstream of an observation method for a low-vacuum (approximately 1 Pa to 3,000 Pa) region. This is because, when a large number of gas molecules existing under low vacuum and electrons having an image signal repeatedly collide against each other, the electrons having image information lose energy thereof in this detection process and thus cannot reach a detector, so that the observation utilizing electrons with higher energy, that is, reflection electrons has been considered as a method which enables easier observation. The type of a material of an observation sample, more specifically, an atomic number effect thereof remarkably appears on the obtained image, and hence this method is frequently utilized at present for, particularly, surface observation and analysis of the surface in the materials field. Moreover, a high scanning speed (TV-Scan and the like) can be sufficiently dealt with, irrespective of a high vacuum or a low vacuum, and this is one of the reasons why this method has been utilized mainly for a detector.
Meanwhile, in recent years, a detection method of utilizing secondary electrons with small electron energy has been actively studied. For example, Patent Literatures 1, 2, and 3 exist. According to a large part of the existing methods, an electrode is placed in advance above a sample, and cascade amplification is utilized in which secondary electrons generated from the sample are accelerated so as to repeatedly collide for amplification against gas molecules existing inside of a sample chamber.
Such a method is known as roughly two types of detection methods. One is an electron current detection method of detecting the amplified secondary electrons themselves, and the other is an ion detection method of detecting positive ions which are generated when the secondary electrons and the gas molecules collide against each other.
As representative examples of the conventional technologies, Patent Literature 1 can be cited for the electron current method, and Patent Literatures 2 and 3 can be cited for the ion current method.
Both of the images obtained according to the two methods closely resemble a high-vacuum secondary electron image because a basic signal source is the secondary electrons from the observation sample, and it is possible to obtain an image having properties different from those of a reflection electron image, that is, an image having information on an extreme surface of the observation sample.